Perspective pubs.acs.org/JPCL Electrode−Electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights † ‡ ‡ § † ‡ † ‡ ⊥ † ‡ Magali Gauthier, , Thomas J. Carney, , Alexis Grimaud, , Livia Giordano, , , Nir Pour, , † ‡ ‡ ∥ ∇ ¶ ¶ Hao-Hsun Chang, , David P. Fenning, , Simon F. Lux, Odysseas Paschos, Christoph Bauer, ¶ ¶ ¶ † ‡ § ∥ Filippo Maglia, Saskia Lupart, Peter Lamp, and Yang Shao-Horn*, , , , † ‡ § Research Laboratory of Electronics, Electrochemical Energy Laboratory, Department of Materials Science & Engineering, and ∥ Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ⊥ Dipartimento di Scienza dei Materiali, Universitàdi Milano-Bicocca, Via Roberto Cozzi 55, 20125 Milan, Italy ∇ BMW Group Technology Office USA, 2606 Bayshore Parkway, Mountain View, California 94043, United States ¶ BMW Group, Petuelring 130, 80788 München, Germany *S Supporting Information ABSTRACT: Understanding reactions at the electrode/electrolyte interface (EEI) is essential to developing strategies to enhance cycle life and safety of lithium batteries. Despite research in the past four decades, there is still limited understanding by what means different components are formed at the EEI and how they influence EEI layer properties. We review findings used to establish the well-known mosaic structure model for the EEI (often referred to as solid electrolyte interphase or SEI) on negative electrodes including lithium, graphite, tin, and silicon. Much less under- standing exists for EEI layers for positive electrodes. High-capacity Li-rich · − layered oxides yLi2−xMnO3 (1 y)Li1−xMO2, which can generate highly reactive species toward the electrolyte via oxygen anion redox, highlight the critical need to understand reactions with the electrolyte and EEI layers for advanced positive electrodes. Recent advances in in situ characterization of well-defined electrode surfaces can provide mechanistic insights and strategies to tailor EEI layer composition and properties. − he use of lithium-ion (Li-ion) batteries to power hybrid, to form the well-known solid electrolyte interphase (SEI).26 32 T plug-in hybrid, and electrical vehicles in recent years Li+-conducting and electronically insulating SEI26,31 can calls for greater battery cycle life and safety characteristics than prevent continuous electrolyte reduction and lithium consump- − their applications in electronic devices. Li-ion batteries operate tion during cycling.33 36 However, the formation of nonuniform by shuttling Li+ and electrons between negative and positive SEI can result in nonuniform lithium deposition and formation − − electrode host structures,1 5 which are separated by a separator of lithium dendrites,37 39 which can lead to battery internal filled with an aprotic electrolyte. Negative electrodes operate short and failure.37 The electrolyte oxidation on the positive + at potentials close to 0 VLi (Li /Li) storing lithium near the electrodes and the composition and properties of the EEI neutral valence state,4,6 whereas positive electrodes normally layer, if any, are less understood than the SEI on negative 5,7−10 40−43 store lithium ions at potentials greater than 3 VLi. Aprotic electrodes. Recent advances in high-capacity positive electrolytes can be thermodynamically unstable against materials, which can generate highly reactive oxygen spe- − 44−49 reduction at negative electrodes11 15 and against oxidation at cies, highlight the need to study EEI layers on their − the positive electrodes.16 22 The formation of a stable surfaces. Such understanding is key to mitigate the electrolyte electrode−electrolyte interface (EEI) layer, conductive to Li+ reactivity with positive electrodes, which can affect battery life but electronically insulating, is critical to ensure high Coulombic and safety as exothermic reactions of positive electrodes with and voltage efficiency,23 cycle life,23,24 and safety24,25 over the flammable electrolytes lead to substantial heat generation, 50−53 course of battery lifetime for vehicle applications.25 Under- triggering thermal runaway. standing electrochemical and chemical reactions between the Progress in understanding the EEI layers on Li-ion negative electrode and the electrolyte, which influence the composition, and positive electrodes has been achieved using techniques such microstructure, and properties of EEI layers formed, is crucial in developing stable and efficient Li-ion batteries. Received: August 7, 2015 Carbonate-based electrolytes commonly used in Li-ion Accepted: October 29, 2015 batteries are reduced on negative electrodes such as graphite © XXXX American Chemical Society 4653 DOI: 10.1021/acs.jpclett.5b01727 J. Phys. Chem. Lett. 2015, 6, 4653−4672 The Journal of Physical Chemistry Letters Perspective − as X-ray photoelectron spectroscopy (XPS),40,54 63 Fourier formation and composition and how its properties contribute transform infrared spectroscopy (FTIR),11,29,41,54,64,65 X-ray to battery performance. − diffraction (XRD),54,66 atomic force microscopy (AFM),67 69 Thermodynamic Considerations of Electrolyte Instability. The − and scanning tunneling microscopy (STM)70 72 However, difference between the Fermi level of the electrode and the − − − although significant work26 28,32,40,41,46,56 58,62,63,65,69,73 83 has highest occupied molecular orbital (HOMO) or the lowest studied the EEI layer, much remains to be understood at the unoccupied molecular orbital (LUMO) levels of the electrolyte fundamental level. Despite considerable research in the past few governs the thermodynamic stability of the electrolyte on decades, the SEI is still a puzzle to be solved due to a lack of the electrode and the driving force to form the EEI layer.2,92 questioning of the past results and an excessive confidence in the A negative electrode with a Fermi level higher in energy than 77 miraculous properties of the SEI. There is still limited under- the LUMO of the electrolyte will have a driving force to reduce standing on what EEI layers consist of, by what mechanisms they the electrolyte (Figure 1b).2 On the positive side, there will be a are formed, and how they influence EEI properties and battery driving force for electrolyte oxidation if the Fermi level of the performance. electrode is lower in energy than the HOMO of the electrolyte (Figure 1b).2 Below, we review and align electron energy levels There is still limited understand- associated with lithium insertion in electrodes with estimated ing on what EEI layers consist of, LUMO and HOMO of electrolytes to discuss the thermody- by what mechanisms they are namic driving force for EEI layer formation associated with fl electrolyte instability on the electrode surface. formed, and how they in uence Lithium Storage Electrode Potentials. The electrochemical EEI properties and battery potential of an electrode for lithium storage (the energy level performance. for electron transfer to and from the material, or redox electron energy) corresponds to the average electron energy level at the ff In this Perspective, we first review and discuss thermo- top of the itinerant or valence band. The di erence between the electrochemical potentials of the negative and positive electrodes dynamic considerations of electrolyte reduction and oxidation fi on negative and positive electrodes. Second, we discuss the de nes the thermodynamic cell voltage. Lithium and graphite, historical evolution of the concept of the SEI composition and the most common negative electrodes storing lithium at 0 and ∼ 4,93 0.1 VLi, respectively, have Fermi levels that lie in an itinerant structure on lithium and graphite, which depends on the salt 2 and solvent used in the electrolyte and on the surface termina- band above the estimated LUMO levels of electrolytes, as tion of the electrode. Third, we compare SEI compositions shown in Figure 1a. Similarly, high-capacity alloy electrodes, such 94 95 ∼ − found on lithium and graphite with those reported for alloyed as Sn and Si storing lithium in the range of 0 1VLi,also negative electrodes such as Sn and Si. Fourth, we summarize have Fermi levels above the estimated LUMO levels of aprotic the EEI layer models reported for oxides of positive electrodes, electrolytes. Therefore, there is a thermodynamic driving force where the nucleophilicity of oxide surfaces is correlated with for electrochemical reduction of electrolytes at the surface of EEI layer composition and impedance. In addition, there is a these negative electrodes. Employing surface films at the EEI critical need to understand the electrolyte oxidation with high- (also called SEI), which are stable, electronically insulating, and capacity oxide electrodes that release highly reactive oxygen lithium-ion conducting, can passivate the electrode surface and species, where much learning can derive from knowledge on prevent direct contact of the aprotic electrolyte to the electrode − 84−91 electrolyte instability developed in the Li O2 chemistry. surface. Lastly, we will discuss opportunities and techniques that can Lithium transition metal compounds such as oxides, sulfides, be used to gain mechanistic insights underlying EEI layer and phosphates have electrochemical potentials (Figure 1a,b) Figure 1. (a) Potentials and expected capacities based on full extraction of selected lithium storage materials for Li-ion batteries (details on the − potentials reported from previous work and calculations of capacities can be found in the Supporting Information).4,5,10,93